missing unittests for overlapping dest when using subparsers

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-10-27.16:36:51.812>
labels = []
title = 'missing unittests for overlapping dest when using subparsers'
updated_at = <Date 2021-10-27.16:36:51.812>
user = 'https://bugs.python.org/paulj3'

bugs.python.org fields:

activity = <Date 2021-10-27.16:36:51.812>
actor = 'paul.j3'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-10-27.16:36:51.812>
creator = 'paul.j3'
dependencies = []
files = []
hgrepos = []
issue_num = 45631
keywords = []
message_count = 1.0
messages = ['405110']
nosy_count = 3.0
nosy_names = ['rhettinger', 'paul.j3', 'ALSchwalm']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue45631'
versions = []